This invention is related to nondestructive testing methods and, more particularly, to such methods employing the generation and detection of ultrasonic waves.
An increasing concern with efficiency and economy in many areas of modern structural design has motivated more widespread use of nondestructive testing methods. Nondestructive methods are important because of their ability to locate a structural defect at an early stage in the life of the defect, so that the appropriate corrective action, such as removing and replacing the defective component, can be initiated before such a defect leads to a catastrophic failure of a component. Before such testing methods became available, it was necessary to design structural components with the assumption that flaws of a certain size were present in the building materials. This practice led to the need to provide structural components of sufficient size and strength to function properly even when such defects were present. When nondestructive testing measures can be implemented, however, similar structural components may be manufactured and installed at a lower expense by reducing the size, substituting less expensive materials, etc. Flaws and other defects of a magnitude which might cause failure in these less expensive components can then be detected by nondestructive testing methods and remedied before such undesirable consequences occur. In this manner, nondestructive inspection techniques can be utilized to maintain the desired level of reliability in structural designs while at the same time reducing construction costs.
One type of defect which plays a significant role in the prediction of structural failure is a discontinuity, such as a crack, a weld, or a slot, in a material. A number of methods for evaluating such flaws are known in the nondestructive testing art. Some of the known methods which utilize ultrasonic waves, for example, direct an ultrasonic signal toward the side of the discontinuity. The discontinuity will then reflect and refract the ultrasonic wave energy in a characteristic manner. This characteristic interaction has been measured in prior art techniques be detecting a change in the coefficient of the reflected wave, detecting a change in the transmission coefficient of the transmitted wave, or by observing a mode conversion to another type of ultrasonic wave motion. These changes are typically utilized to estimate such parameters as the depth of the discontinuity, which parameters can then be utilized to predict the remaining useful life of the component before a failure occurs due to the defect.
For defects smaller than a certain size, however, such known ultrasonic methods do not exhibit sufficient sensitivity to provide useful information about the defect. Where a thin, shallow slot is present in a plate type of material, for example, the sensitivity of such methods appears to decrease as the square of the ratio between the depth of the slot and the thickness of the plate. Consequently, a need has developed in the art for an ultrasonic nondestructive evaluation method exhibiting sufficient increased sensitivity to be capable of characterizing small discontinuities in an object.